Vertically emitting semiconductor lasers with an external resonator, which are also known as disk lasers or under the designation of VECSEL (Vertical External Cavity Surface Emitting Laser), constitute a new concept for lasers of high power at the same time as high beam quality. Such semiconductor lasers are described, for example, in U.S. Pat. No. 6,327,293, WO 00/25398 and U.S. Pat. No. 5,991,318.
A VECSEL includes a semiconductor body that includes as essential elements a reflector and a quantum well structure as active zone. The term semiconductor body is to be understood here and in the following discussion in such a way that this element consists substantially of semiconductor layers, but this does not exclude the possibility that it can, in particular, also include on its surface dielectric or metallic layers that preferably serve for increasing or decreasing the reflectivity of the surface. The reflector is preferably an epitaxially produced DBR (Distributed Bragg Reflection) mirror on which a quantum well structure is located. This quantum well structure includes periodically arranged quantum wells or groups of quantum wells with barrier layers situated therebetween. A VECSEL also includes an external mirror that forms a resonator with the reflector of the semiconductor body.
A characteristic of the VECSEL is the optical pumping process by a light source outside the semiconductor body, for example by a diode laser. In previously known publications, the absorption of the pumping radiation is desirable to take place in the barrier layers situated between the quantum wells. The absorption of the pumping radiation generates in the barrier layers charge carriers that relax into the quantum wells.
A disadvantage of this type of pumping process is that it is greatly affected by losses. Firstly, not all of the charge carriers released by the pumping radiation will enter the quantum wells, that is to say the entry efficiency is less than 1. A further loss mechanism is the relaxation of charge carriers from excited states lying relatively higher in terms of energy into energetically lower-lying levels of the quantum well. This energy loss, denoted as quantum defect, from higher-energy pump wavelength to the laser wavelength is expressed in the generation of heat that is output to the crystal lattice and thereby heats up the component. The maximum output power is therefore limited by the maximum permissible thermal loading.
A further disadvantage of known VECSEL designs is also that the pumping radiation passes only once through the active zone. Consequently, the absorption per quantum well is low, and so efficient operation of the VECSEL is possible only in structures with a high number of quantum wells. This results in disadvantages such as, for example, absorption losses owing to the large number of layers, boundary surfaces, high pump thresholds, low efficiency and an inhomogeneous pumping of the quantum wells.